The present invention is directed to a process for preparing fluorinated pyridines from chlorinated pyridines. In the present process an improved fluorination reaction between a chlorinated pyridine compound and an alkali metal fluoride is set forth. Products of the reaction are useful as intermediates for the preparation of herbicides or pesticides.
British Patent Specification No. 1,256,082 teaches the reaction of chloropyridines with potassium fluoride in a polar aprotic solvent and in the presence of a small amount of water to effect replacement of chlorine by fluorine. Reaction of pentachloropyridine with anhydrous KF in a 1:5.9 mole ratio in sulpholane and water at 215.degree. C. has given 91% yield of 3,5-dichlorotrifluoropyridine, 6% yield of 3-chlorotetrafluoropyridine, and 3% yield of 3,4,5,6-tetrachlorofluoropyridine.
Reaction of pentachloropyridine with KF in a 1:11 mole ratio of 400.degree. C. for 18 hours in the absence of a solvent has given 84% yield of 3,5-dichloro-2,4,6-trifluoropyridine (J. Chem. Soc. 1964, 3573). Reaction of pentachloropyridine with KF in a 1:9.5 mole ratio at 400.degree. C. for 5 hours in the absence of solvent has given 45% yield of 3,5-dichlorotrifluoropyridine and 19% yield of 3-chlorotetrafluoropyridine (J. Chem. Soc. 1965, 594).